1. Field of the Invention
This invention relates to a semiconductor device, and in particular, to a semiconductor device having Cu multi-layer wirings and an improved pad electrode structure for wire bonding.
2. Description of the Related Art
Conventionally, the pad electrode of an LSI having a Cu wiring is usually provided with an Al cap layer for protecting a Cu pad. The reason for this is that Cu has a lower corrosion and oxidation resistance, compared to Al. In particular, in a product where wire bonding is employed, since it is difficult to perform direct bonding to Cu, the Al cap layer is generally employed.
However, even if the Al cap layer is formed in this manner, it suffers from surface roughening or peeling due to the application of probing thereto, thereby raising various problems, such as the deterioration of the yield of wire bonding or the corrosion of wirings due to the exposure of a Cu pad. Further, depending on the kind of product, the probing may be performed a great number of times on the Al cap layer prior to the application of wire bonding thereto, thus making the aforementioned problem more conspicuous.
As explained above, even if the Al cap layer is formed on the surface of the Cu pad, as is conventionally practiced, there are still problems that, due to mechanical stress, caused by probing, for example, the yield of wire bonding deteriorates or the wiring is corroded.
With a view to solve the aforementioned problems, there has been proposed a structure as shown in cross-section in FIG. 1. Here, a Cu pad and an Al cap layer are not laminated, interposed with an insulating film (an interlayer film), thereby minimizing the effects of probing and wire bonding.
More specifically, as shown in FIG. 1, an interlayer insulating film 42 is formed on the substrate 40 having therein a Cu wiring 41 and a pad portion 41a of the Cu wiring 41 formed in advance. An Al cap layer 44 is formed on this insulating film 42. A plurality of contact holes 43 are formed in this interlayer insulating film 42 which is located over the pad portion 41a, and the Al cap layer 44 is electrically connected via these contact holes 43 with the Cu pad portion 41a. In this structure, since the insulating film 42 is disposed below the Al cap layer 44, it is now possible to inhibit the deterioration of the yield of wire bonding that may be caused by the surface roughening or peeling of the Al cap layer due to the probing, and also to inhibit the generation of corrosion of wiring due to the exposure of the Cu pad.
However, when a contact hole 43 is disposed below the opening of a pad, as shown in the structure of FIG. 1, the interlayer insulating film 42 is caused to isolate between the cap layer 44 and the wiring pad 41a. As a result, there is a problem that the insulating film 42 may be destroyed by mechanical stress caused by the aforementioned probing. Once this insulating film 42 is destroyed, there may be raised a possibility of also damaging the underlying Cu wiring 41.